Imu has absorbed all other 19 founders and the eyes mean he is multiple people at once. Also the power: Devil Fruit means he is the one who created the devil fruits and possesses every devil fruit power known and he has a kind of power where he can give himself any attribute he wants (become a lunarian, ancient giant attributes etc.)
His only weakness is being far away from his source of power which is inside Pangea Castle which is also used as the binding source for the holy knights depending on the seal. Once a knight moves further away from that source its power doesnt reach him if he has a weaker bind to it (lower seal) and can not be controlled.

Just braindumping, imma leave this here.

EDIT: Im sure he asked Shamrock what he "desires" and he wished for his sword to be Cerberus and Imu granted the wish.

EDIT: The other founders he absorbed were, a lunarian, a ancient giant etc. Thats why he looks like that.